April 2014- In 1961 Cessna debuted its first turbocharged twin-engine aircraft: the 320 Skyknight. Cessna engineers had been experimenting with turbocharging for a number of years using the 310 airframe. The prototypes—fitted with the new Continental Motors TSIO-470 engines—were subjected to flight testing at Continental and at Cessna’s headquarters in Wichita.

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The engineers and test pilots faced a couple of unique challenges. Here’s how Daryl Murphy, longtime aviation writer and author of “The Planes of Wichita: The People and the Aircraft of the Air Capital,” describes it: “Since the one unique advantage of turbocharging is its ability to develop power at all altitudes, the biggest challenge for test pilots was engine cooling, because indicated airspeed is coupled with thinner air at the more desirable higher levels.”

Murphy continues, “A second problem was designing a combustion air inlet system with minimum restriction. The 310’s sheltered inlet screen within the engine compartment needed to be replaced with an external inlet that could retain almost full impact pressure. The solution was an inlet in the wing leading edge just inboard of each nacelle. From there, the air was ducted to the compressor, which was located behind the engine.”

Certified on May 24, 1961 with two Continental TSIO-470-B engines, the Skyknight was sold as a 1962 model. The aircraft was similar in appearance to the 310G but had a different nacelle design and slightly different cabin height.

Cessna introduced the new model in the May 1961 Cessquire magazine. Here is an excerpt:

The Skyknight is a turbocharged twin-engine airplane with speeds up to 260 miles per hour. It is a fast, roomy, five-passenger airplane which will occupy the top position in the Cessna 1962 line….

With turbocharged engines, the Skyknight will have available the full 260 horsepower of its engines to 16,000 feet, providing greater takeoff and single-engine safety at high altitudes. Single-engine service ceiling is in excess of 17,000 feet.

The Skyknight has been specifically designed to fly long distances at high altitudes topping weather conditions over 27,000 feet, and will offer business executives high-speed flight in comfort, with outstanding visibility at altitudes where flying is at its finest. A combination of four seating arrangements, including a lounge, will be available.

While it shared its good looks with the 310, the 320 was a step up in performance; in fact, it was the reputed to be the fastest GA aircraft of its day. Besides the airplane’s top speed of 260 mph at 16,000 feet, it had a maximum cruising speed of 244 mph at 24,000 feet—17 percent higher than the comparable 310 model.

The 320’s gross weight was only about 100 pounds more than the 310, and it had similar fuel consumption at economy settings.

A Cessna Flyer Association member and owner of a 1967 320E reported that at 60 percent cruise power, the light twin cruised at 180 knots at 10,000 feet; 190 knots at 15,000 feet; and 200 knots at 20,000 feet—all the while consuming fuel at the rate of 28 gph. Sea level takeoffs took less than 1,500 feet of runway, landings at sea level took less than 2,000 feet.

Canted tiptanks replaced the tuna-shaped tanks in 1963, along with an increased gross weight of 5,200 pounds and addition of nacelle baggage lockers the next year. For the 1966 320D, 285 hp TSIO-520-B engines boosted top speed to 275 mph, service ceiling to 29,000 feet, and allowed an increase in gross weight to 5,300 pounds.

Only 577 Skyknights were built over its seven-year run. By all rights, the performance and relative economy of the twin should have made it more popular.

Perhaps buyers were frightened away by the extra care and maintenance the turbocharged engines presented. The original Continental TSIO-470Bs were carrying a recommended TBO of only 1,400 hours compared to 1,500 hours for the normally aspirated 470s of the time. Service life of the turbochargers was variable and highly susceptible to operator misuse.

Whatever the reason, the 320 was valuable in developing turbocharged systems and components. Nearly every improvement that was made for the 320 found its way into the 310, and not coincidentally, the Turbo 310—with 320 wings and engine—was introduced in 1969.

September 2005- Turbo-supercharging was patented in 1906, but the system was used only sporadically because of the need of extreme temperature-resistant component materials. During World War II, it found its niche on high-speed, high-altitude bombers, and by the 1940s and ‘50s many large airliners utilized turbocharged Wright and Pratt & Whitney piston engines.

By 1960, the piston engine had been relegated to General Aviation power, and although several aftermarket units were beginning to come on the market, engine maker Continental knew that to ensure long-term reliability, many components would have to be engineered as part of an integral engine design (hence the “TS” in Continental nomenclature stands for Turbo System).

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Cessna had begun experimenting with turbocharging several years after the Model 310 debuted in 1954 and built two prototypes that were designated C-320. One went to Continental Motors in Muskegon, Mich. for test flying with a newly-developed turbocharged version of the 310’s 260 hp IO-470 engines and the other remained in Wichita for flight testing.

Since the one unique advantage of turbocharging is its ability to develop power at all altitudes, the biggest challenge for test pilots was engine cooling, because indicated airspeed is coupled with thinner air at the more desirable higher levels.

A second problem was designing a combustion air inlet system with minimum restriction. The 310’s sheltered inlet screen within the engine compartment was replaced with an external inlet that could retain almost full impact pressure. The solution was an inlet in the wing leading edge just inboard of each nacelle. From there, the air was ducted to the compressor, which was located behind the engine.

With the requirement of extra exhaust manifolding and the associated heat generation, testing at altitudes of 25,000 feet and above, pilots also worried about the possibility of in-flight fires, but nothing ever happened to indicate it would be a problem (someone joked there possibly was not enough oxygen at that altitude to sustain a fire).

Introduced to the public as a 1962 model, the 320 Skyknight was a turbocharged version of the 310. The notable physical exceptions between it and the 310G were a different nacelle design and a slightly different cabin height.

In the performance area, however, the 320 was by far the fastest GA aircraft of its day. With a gross weight of 4,990 pounds, it had a top speed of 260 mph at 16,000 feet and a maximum cruising speed of 244 mph at 24,000 feet.

Although Cessna had always made allusions about the use of “business” aircraft, the Skyknight marked one of the first business aircraft exposures within the context of the entertainment medium. The 320 was a co-star on an early 1960s weekly sitcom called “The Executive,” starring Walter Brennan.

At a $69,950 base price, the buyer was paying an extra $10,000 premium (over the 310) for 20 mph more speed, but 110 customers stepped up and plunked down their money in its first year.

It was an aerial hot rod, but not that many flew it routinely at its efficient cruising altitudes because of the new experience of needing oxygen and resupplying the tanks, and engine maintenance was a consideration because more horsepower was being produced at high altitudes where cooling was often marginal.

The Continental TSIO-470Bs were carrying a recommended TBO of only 1,400 hours compared to 1,500 hours for the normally aspirated 470s of the time.

Service life of the turbochargers was variable. Those who impatiently shut down without a spin-down period often experienced a “coking” on the turbine bearing caused by a loss of oil pressure when the mixture knob was pulled.

Canted tip tanks replaced the tuna-shaped tanks in 1963, along with an increased gross weight of 5,200 pounds and addition of nacelle baggage lockers the next year.

In the 1966 320D, 285 hp TSIO-520-B engines boosted top speed to 275 mph, service ceiling to 29,300 feet and allowed an increase in gross weight to 5,300 pounds.

While the 320 may have been hot in the skies (and under the cowl), it never burned up the marketplace. During its 1962-1968 life, only 577 units were sold.

But it had served its purpose in developing systems and components for its sibling the Model 310. Nearly every improvement that was made for the 320 found its way into the 310, and not coincidentally, the Turbo 31 0—with 320 wings and engine—was introduced in 1969.